The present invention relates to an array antenna, in particular, relates to an adaptive direction beam control system of such an antenna for avoiding interference due to multipath propagation.
An adaptive array antenna adjusts directivity of an antenna adaptively so that the antenna receives the most preferable wave among a plurality of waves which reach the antenna for avoiding effect of multipath propagation. The most preferable wave may be a direct wave which is received directly from a transmit point, a wave which has the least delay time, or a wave which has the highest receive level.
FIG. 2 shows a conventional adaptive array antenna. In the figure, the numeral 1 (1-1 through 1-N) shows antenna elements, 4 (4-1 through 4-N) shows variable complex weight circuits, 5 is a combiner, 6 is an error detector, and 7 is a complex weight control. The receive signal X.sub.n (t) by the n'th antenna element 1-n is applied to the n'th variable complex weight circuit 4-n which adjusts amplitude and phase of the receive signal by multiplying the receive signal and the complex weight W.sub.n. The product W.sub.n X.sub.n (t) is applied to the combiner 5 which combines the products of all the antenna elements, and provides the output signal Y(t) of the adaptive array antenna. The error detection circuit 6 measures the error between the evaluation function e(y) which depends upon the output signal Y(t) and the preferable value. The error signal thus measured is applied to the weight control 7 which adjusts weights W.sub.n (t) for each adaptive complex weight circuits 4-1 through 4-N so that the error becomes small.
Thus, an adaptive array antenna provides a receive signal through operation of feedback loop.
The operation of a conventional adaptive array antenna for avoiding effect of multipath interference is described in accordance with FIGS. 3 through 4. In those figures, the numeral 9 is a transmit point, 10 is a receive point, #1 through #5 are receive waves at the receive point 10 from the transmit point 9, .theta..sub.1 through .theta..sub.5 show receive angles of receive waves #1 through #5 at the receive point 10, T.sub.1 through T.sub.5 are delay times of the waves #1 through #5, respectively, at the receive point 10. The receive waves may be either direct wave #1, or reflected waves #2 through #5 reflected by the reflected bodies B2 through B5.
FIG. 3 shows an example of a multi-path propagation. The signal wave from the transmit point 9 reaches the receive point 10 through a plurality of propagation paths some of which reflect by buildings B2 through B5.
FIG. 4 shows the receive angle (FIG. 4A) and the delay time (FIG. 4B) of the receive signals #1 through #5 at the receive point 10.
In a prior art, the directivity of an antenna is adjusted so that the antenna receives one signal selected from a direct wave, a wave having the least delay time, or the highest level wave. In the example, the wave #1 is a direct wave, and has the highest level, and therefore, the directivity is adjusted so that it receives only the wave #1. The antenna in this case has only one beam.
When wideband digital communication is carried out in a large city, the relative delay time of the receive waves extends several times as long as the symbol interval of transmission signal. Further, it might be impossible sometimes to receive a direct wave. Although the receive G/T (Gain to Thermal) ratio or C/N (carrier to noise ratio) must be high in wideband communication, if all the receive signals are combined as they are in order to provide high receive level, the receive signal would be distorted when delay time extends widely.
Further, when it is impossible to receive a direct wave, if we receive a wave which has the least delay time, it might not provide enough receive level, since that wave is not always the highest wave of all the waves.
Further, if we select only one wave, other waves are not used, and therefore, the energy efficienty is low, since other waves might have the similar level to or even higher level than that of the selected wave, and have the same information as that the selected wave.
Thus, a prior adaptive array antenna must have sharp directivity in direction of a desired wave component, and must have so many antenna elements.